1. Field of the Invention
The present invention relates to devices for determining radiant heat flux.
2. Description of the Related Art
When a mass is placed in an enclosure whose walls are at a temperature above that of the mass, the temperature of the mass will increase even if the enclosure is evacuated. The process by which heat is transferred from the enclosure to the mass by virtue of the temperature difference between the enclosure and the mass, without the aid of any intervening medium, is called thermal radiation. The emission of thermal radiation is governed by the temperature of the enclosure.
Direct-fired heaters are commonly used in many applications. Direct-fired heaters have an interior combustion chamber in which an ignition source, fuel and oxygen react to form a flame. The flame is commonly formed near the bottom of the heater and the combustion products exit the top of the heater through a flue. Fluid tubes are positioned along walls of the heater. The difference between density of the hot gases inside the heater and the density of cooler ambient air creates a vacuum pressure inside the heater. A vacuum pressure is a pressure below the ambient atmospheric pressure.
Radiant heat flux is the radiant heat transfer per unit area across a control surface. In direct-fired heaters, it is desirable to measure the radiant heat flux rate because locally high flux rates may shorten equipment life and increase the need for frequent cleaning of the equipment. High flux rates also cause carbon deposits to form in a fluid film at the inside wall of fluid tubes positioned inside the heater.
In order to determine the heat flux, a cooling mechanism is needed to maintain a probe tip cooler than a static equilibrium temperature inside the heater, so that a net heat transfer will occur through the tip. In the past, in-situ heat flux probes have been cooled by pumping or compressing a fluid (gas or liquid) through the probe. These devices, however, have several disadvantages. One disadvantage is these devices require moving parts such as pumps, metering devices and valves. Another disadvantage is these devices require an external supply to power the pump or compressor that transports the cooling fluid.